Nuclear medicine imaging modalities are injecting radioisotopes into a bio-body, and detecting the gamma rays emitted by the radioisotopes so that the distribution of the radioisotopes in the bio-body can be known. Accordingly, the function of tissues and organs can be diagnosed.
A scintillation detector can be used to detect ionizing radiation such as gamma rays and is composed of scintillation crystals, photomultiplier tubes (PMTs) and electronic circuits. As the gamma rays and scintillation crystals interact to emit scintillation photons, the scintillation photons are converted into electrons and amplified by PMTs. Then the electronic circuits perform signal processing. During the imaging process, the acquired signals are influenced by the different efficiency of scintillation crystals, PMTs, and electronic circuits. To minimize the influence and improve the imaging quality, the detection efficiency of the scintillation detector or imaging apparatuses thereof such as gamma cameras, positron imaging apparatus, positron emission tomography (PET), single photon imaging apparatus, single photon emission computed tomography (SPECT), etc. has to be calibrated so as to avoid error information due to the difference in detection efficiency between crystals.
The method of crystal-level detection efficiency calibration is realized by performing an imaging process to a uniform radiation source by the scintillation detector or imaging apparatuses thereof. Then the interaction signals of the crystals with the gamma rays are accumulated so as to calculate and calibrate the detection efficiency of the crystals. Conventionally, the ring-shaped imaging system such as positron imaging apparatus for imaging the human body and the rotary single photon imaging apparatus with parallel-hole or approximately parallel-hole collimators can generally uses point sources, uniform cylindrical sources or rotary planar sources for efficiency calibration with good results. With the trend of personalized medicine, specialized imaging apparatuses with specific functions has become more popular. Therefore, the non ring-shaped imaging system and the single photon imaging system with non parallel-hole collimator (such as pinhole or multi-pinhole collimator) have become more and more important. Since the non ring-shaped system and ring-shaped system, parallel-hole collimator and non parallel-hole collimator are geometrically differently designed, it is important to develop a detection efficiency calibration technology that is applicable to non ring-shaped system and non parallel-hole collimator system.
For example, FIG. 1 is a schematic diagram of a conventional ring-shaped imaging system. As shown in FIG. 1, the conventional ring-shaped imaging system 9 comprises a ring-shaped detection module 94 composed of a plurality of crystals 92 (only one crystal is indicated as an example). The detection efficiency of the ring-shaped imaging system 9 is calibrated with the gamma rays 91 generated by a gamma-ray source 90 being incident vertically onto the crystal 92 so that the crystal penetration effect does not bring forth significant influence. Therefore, in such a conventional ring-shaped detection method, the influence caused by the crystal penetration effect is not taken into consideration when calculating the crystal efficiency. However, in a non ring-shaped system, as shown in FIG. 2, since the gamma rays 91 generated by the gamma-ray source 90 may be incident onto the crystal with a larger incoming angle, the influence caused by the crystal penetration effect is not negligible. The crystal penetration effect is correlated to the interaction between the gamma rays and the crystals. It is not necessarily that scintillation photons are generated immediately once the gamma rays are incident on the crystals. It is more probable that scintillation photons are generated after the gamma rays have entered the crystals for a certain depth. Therefore, the crystal that generates scintillation photons is not necessarily the first crystal that receives the gamma rays. For example, in FIG. 2, the gamma ray 91 enters the crystal C on the surface thereof and is detected in the crystal A due to penetration. Therefore, if the influence caused by the crystal penetration effect is not calibrated, misjudgment on the position of the gamma ray happens. It follows that the difference of the efficiency between the crystals can not be calibrated correctly, which results in poor resolution and contrast of the imaging result to adversely affect the imaging quality and worsen the difficulty in diagnosis.